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    <title>Document</title>
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<script>

    function convertToCase(str) {
        let arr = str.split('');
        for(let i = 0; i < arr.length; i++){
            arr[i] = arr[i].toUpperCase() == arr[i] ? arr[i].toLowerCase() : arr[i].toUpperCase();
        }
        return arr.join('');
    }

    let str = 'asdASD';
    console.log(convertToCase(str));

</script>